1. Field of the Invention
The present invention relates to a deinking method for deinking waste papers such as news papers, leaflets and magazines to reclaim them. More particularly, it relates to a deinking method excellent in workability by which an excellent deinked pulp having a higher whiteness and a less residual ink spots is obtained at a high yield.
2. Description of the Related Art
Waste papers are reclaimed by liberating ink from the waste papers by a deinking treatment to prepare reclaimed pulp and manufacturing reclaimed paper from the reclaimed pulp. The conventional deinking method generally comprises a step of liberating ink from waste papers and another step of rejecting the liberated ink.
More specially, the deinking method comprises, as main steps,
(1) a step of pulping (disintegrating) waste papers, PA0 (2) a step of aging, i.e., leaving the disintegrated paper as it is, PA0 (3) a step of flotation, and PA0 (4) a step of washing.
That is, in deinking treatment, ink bound to the fibers of waste papers is physically and chemically (or biochemically) liberated therefrom to thereby separate the ink from the fibers. Thus, reclaimed pulp is obtained.
In effecting deinking treatment, a deinking agent comprising a surfactant such as a polyoxyethyl alkyl ether and a fatty acid is generally used in any of the above-described steps, for the purposes of liberating ink from waste papers or generating foams to which the liberated ink will adhere.
In recent years, the properties of waste papers which are used as raw materials have been varied because of an increase in a demand for a beautiful and stable printing and a progress in printing techniques. In addition, degrees of binding of ink to paper are varied, including loose ones and tight ones. Under these circumstances, it is demanded to improve the quality and the recovery (i.e., the productivity) of deinked pulp. In order to attain such a purpose, various proposals are offered from various fields including deinking processes, deinking agents and apparatuses used for deinking.
In conventional deinking methods, the pulping step and the flotation step are usually carried out at an alkaline pH of exceeding 9, and in an extremely few cases, at a pH of 9 or less. Deinking methods wherein the pulping step and the flotation step are carried out at a pH of 9 or less are disclosed in Japanese Patent Publication-A Nos. 54-23705 and 59-53532, and U.S. Pat. No. 4,043,908 (published on Aug. 23, 1977; Assignees: Kemanord Aktiebolag and Sala International AB). Specially, Japanese Patent Publication-A No. 54-23705 discloses a method wherein an aluminum salt or white water is added to a waste paper dispersion, the pH of the dispersion is adjusted to a pH ranging from 4 to 7, a polyelectrolyte is added to the dispersion, and then the flotation is effected. Japanese Patent Publication-A No. 59-53532 discloses that a specific alkylamine polyether, which behave as an amine at a basic pH and as an quaternary ammonium at an acidic pH, contributes to the appearances of foam performances required during the pulping step, deinking step of washing-system and flotation step, respectively. U.S. Pat. No. 4,043,908 discloses a method wherein an ethylene oxide adduct of an amine is used for liberating ink from pulp, and the disintegration of waste papers and the liberation of ink therefrom is carried out at a high pH to thereby introduce the ink into micelles, and then the pH of the system is reduced to break the micelles, thereby separating (precipitating) the ink from the pulp, and the ink is removed out of the system by flotation.
According to the method disclosed in Japanese Patent Publication-A No. 54-23705, however, the lathering power is markedly high, and therefore a decrease in the recovery of pulp and an increase in the amount of foams to be rejected (or discharged) during flotation step occur. Such an excess generation of foams is not preferred for flotation and brings about reductions in productivity and quality. The alkylamine polyether disclosed in Japanese Patent Publication-A No. 59-53532 is inferior to conventional polyoxyethylene alkyl ether type deinking agents in the ink-liberating performance and therefore can not provide a pulp having a high whiteness. Further, the ink liberated from pulp with the use of such an alkylamine polyether is in the form of very fine drops and does not have an optimum size suitable for the adsorption to the foam during flotation. Accordingly, the ink can not sufficiently be removed during flotation. The method disclosed in U.S. Pat. No. 4,043,908 does not attain sufficient deinking. The reason is not necessarily clarified. but it is considered to be due to when an ethylene oxide adduct of an amine is used as the deinking agent in the disintegrating step wherein ink is liberated from pulp, the ink is not sufficiently liberated therefrom. and that when the pH of the system is reduced for flotation, the ink which is dispersed in the system or dissolved in the micelles once again adheres to the pulp.
WO 94/28237 teaches a flotation deinking process which comprises the steps of:
1. forming a pulping slurry of printed media and water PA1 2. adding to the slurry (i) a specific non-ionic surfactant of the formula RO[(AO).sub.n -R.sup.o ]m; and (ii) a fatty acid or salt thereof of the formula R.sup.5 COOM wherein R.sup.5 is a linear or branched alkyl (alkenyl) and M is hydrogen or a counterion; and PA1 3. subjecting the slurry to flotation deinking in a flotation cell characterized by: PA1 1. forming a pulp from the waste paper in a aqueous medium that has a pH of between 6 and 9 and that is substantially free of dissolved phosphate and includes an ink-dispersing amount of dispersing agent; and PA1 2. separating the ink from the pulp (e.g., by flotation, page 6 line 9). PA1 nonionic surfactant (A): a reaction product obtained by adding an alkylene oxide to a mixture of an oil & fat and an alcohol, PA1 nonionic surfactant (B): a compound represented by the formula: RCOO(AO).sub.m R' (wherein R represents an alkyl or alkenyl group having 7 to 23 carbon atoms; R' represents a hydrogen atom, an alkyl group having 1 to 22 carbon atoms, an alkenyl group having 2 to 22 carbon atoms, or an acyl group having 2 to 22 carbon atoms; AO represents an oxyalkylene group having 2 to 4 carbon atoms; and m is an integer of 1 or more), PA1 nonionic surfactant (C): a compound represented by the formula: RO(AO).sub.n H (wherein R represents an alkyl or alkenyl group having 8 to 24 carbon atoms; AO represents an oxyalkylene group having 2 to 4 carbon atoms; and n is an integer of 1 or more), and PA1 nonionic surfactant (D): a reaction product obtained by adding an alkylene oxide to a polyvalent carboxylic acid or an acid anhydride thereof; or a reaction product obtained by adding an alkylene oxide to a mixture of a polyvalent carboxylic acid or an acid anhydride thereof, and alcohol.
(A) using a specific printed media PA2 (B) adding a water-soluble cationic polymer to the slurry prior to the flotation deinking.
During the pulping step the pH is maintained at about 7.5 to 11. After the slurry exists the pulper it is diluted to about 1 weight % solids. A specific pH reduction step is not taught.
WO 93/21376 describes a process of deinking waste paper comprising:
In this process preferably a surfactant, generally a non-ionic surfactant such as an ethoxylated phenol or fatty alcohol may be used.